1. Field of the Invention
This invention pertains to a ballast circuit for operating an HID lamp and more specifically to such a ballast circuit capable of operating either in a reduced energy standby mode or a full light output mode.
2. Description of the Prior Art
High intensity discharge (HID) lamps include mercury vapor lamps, metal halide lamps, and high pressure sodium lamps, each requiring a ballast circuit operating therewith to accommodate to the pre-strike and post-strike conditions of the lamp. Conventionally magnetic transformer ballasts have been employed to provide the voltage and current compensation required. Even the most sophisticated circuits, however, cannot instantaneously cause such a lamp to go from a cold start to a high level operation and, therefore, unlike incandescent lamps or even fairly rapidly starting low pressure discharge lamps, for example, fluorescent lamps, if the operating requirements are such as to make desirable a fast full light output condition, it is required to keep such lamps operating at a dimming output level to begin with. Moreover, a dimming condition is often desirable in any event to provide emergency lighting to the areas serviced by the lamps.
Dimming controls suitable for non-HID lamps are notoriously unsuited for HID lamps that must have a continuous voltage and current condition maintained and without prolonged phase reversals, characteristic of many of such devices, since this will cause HID lamps to extinguish. Extinguishment of an HID lamp requires a subsequent long start up time, as discussed above. Many other techniques employed are not efficient in that although providing reduced power to the lamp, there is no reduction of power overall, excess power not directed to the lamp being wasted in heat loss or the like. Nevertheless, various techniques have been employed to provide dimming, perhaps the most successful being the employment of careful removal of some of the applied voltage to a lamp each half cycle without causing lamp extinguishment. A circuit that does this is shown in U.S. Pat. No. 4,482,844, Schweer, et al., commonly assigned.
However, a range of dimming operations such as shown in the '844-type circuits is usually not required, such circuit having a large number of components to accomplish this feat. It is usually satisfactory for a ballast circuit to operate either at a full light output level or at a reduced output light level. Moreover, although all or nearly-all electronic ballasts have been designed and made available in recent years, it is still recognized that constant wattage auto-transformer (CWA) ballasts and regulated lag ballasts are still highly favored for their dependable operation. Heretofore, a relatively simple and efficient bi-level ballast circuit operating with a conventional magnetic transformer ballast has not been available.
Therefore, it is a feature of the present invention to provide an improved bi-level operating ballast circuit for operating with a magnetic transformer ballast to efficiently operate an HID lamp at a reduced energy, standby mode and alternatively to operate such lamp at a high, full light level mode.
It is another feature of the present invention to provide an improved bi-level ballast circuit HID operation including controlled switching that occurs at zero voltage crossings, thereby minimizing disruptive, often harmful results.